Mr. Harris brings histrionic splendor and a commanding presence to the role of the legendary monarch who struggled to instill an uncouth people with the ideals of chivalry, justice, and civilization. Making his New York stage debut, the British star proves more eloquent in speech than in song. His singing voice is not for all registers.

Meg Bussert's sweetly tentative Guenevere is a fairy-tale princess, a little girl lost among the tempestuous emotions of a legendary romantic triangle. But when Miss Bussert sings, she is every inch and note an operetta queen. As the self-righteously stiff-necked Lancelot, Richard Muenz has a baritone virility to challenge all comers. In addition, he plays the role with an undertone of wit too often lacking in the ponderous production staged by Frank Dunlop.

As always, ''Camelot'' is a contest between the enchanting Loewe-Lerner songs and a libretto that can be heavy going. All the more reason to welcome the comic disarray of Barrie Ingham's King Pellinore, the White Knight of the Arthurian court, and the orotund reverberance of James Valentine's Merlyn. Jim Backus's malign Mordred sneers like the born troublemaker he is, while the flaxen-haired Tom of Thor Fields brings a grace note of youthful hopefulness to the tender finale.

The musical performance conducted by Franz Allers includes a singing chorus that would do any kingdom proud. Buddy Schwab's courtly dances are prettily performed. Desmond Heeley's settings range from the dark gilt of the castle interiors to the tangled Arthur Rackham-esque undergrowth of the forest scenes. Under Thomas Skelton's lighting, Camelot can still seem at times like the wistfully glimpsed neverland of romantic imagining, that enchanted spot for happy ever-aftering. After the Prize.

Play by Fay Weldon. Directed by Steven Robman.

The articulate British import being given its world premiere at the Marymount Manhattan Theater marked the American stage debut of Fay Weldon. A prolific playwright and novelist, Miss Weldon's credits include a prizewinning segment of ''Upstairs, Downstairs'' and a five-part adaptation of Jane Austen's ''Pride and Prejudice,'' both written for BBC television and broadcast in the United States. ''After the Prize'' should not, however, raise the hopes of Phoenix Theater subscribers too high. Its overall effect is rather dispiriting.

''After the Prize'' tells what happens when a physicist eccentrically named Wasp (Veronica Castang) wins the Nobel Prize. A prolonged first scene of marital bickering indicates the potentially damaging effects of the (STR)88,000 award on Wasp's humdrum marriage to a resentful fellow scientist, Edwin (John Horton), and to her general emotional state. Wasp is coolly detached and monstrously self-centered.

Miss Weldon seems sincerely intent on making some serious observations about male and female roles at a time of shifting opportunities, values, and responsibilities. While the play appears at least superficially antifeminist, the author's deeper purpose may be to warn against the dangers of careerism without humanity and intellect without heart. In any case, this tedious conversation piece scarcely rivets the attention.

To the extent possible, the talented cast directed by Steven Robman plays ''After the Prize'' with intelligence and feeling.